docs: def get_field_value(self, instance, field_name):
          """
          Given an instance, and the name of an attribute, returns the value
          of that attribute on the instance.
  
          Default behavior will map the ``percent`` attribute to ``id``.
          """
          # XXX: can we come up w/ a better API?
          # Ensure we map ``percent`` to the ``id`` column
          if field_name == 'percent':
              field_name = 'id'
          value = getattr(instance, field_name)
          if callable(value):
              value = value()
          return value